Chapter 4 of 6
Once the fields are placed and field blocking is set, we must calculate the dose and evaluate the dose distribution.
Prescribe to CalcPt
For whole brain radiotherapy (WBRT), we prescribe the dose to the calculation point, which is typically located midline where the head’s separation is widest.
Review Dose Distribution
We will then review the dose distribution for coverage and conformity. Additional adjustments to the field blocks can be made if needed at this stage.
Plan Normalization
Normalize the plan to the calculation point and prescribe. To optimize brain coverage, the plan is adjusted by a few percentage points. Here is an example of plan normalization to a calculation point.
Field Energy
Lower energy (6x) photons are often used to cover the entire brain tissue. Higher energy (15x) photons are more penetrating; however, the skin-sparing effect can lead to underdosing of superficial brain tissue and the skull. The image shows a dose distribution comparison of the two energies.
Field Weighting
To enhance the distribution, you can adjust the field weighting to create a more even dose distribution.
Summary
Next, we will evaluate the plan to determine if it achieves the desired dose distribution. We will also review the maximum dose of the plan and organs at risk (OAR) dose statistics.
Next: Plan Evaluation